Aphron-containing well drilling and servicing fluids

ABSTRACT

The invention provides a method and fluid for drilling or servicing a well in a subterranean formation containing lost circulation zones or depleted, low pressure reservoirs. The fluid comprises an aqueous liquid having dispersed therein a polymer which increases the low shear rate viscosity of the fluid to the extent that the thixotropic index of the fluid is at least about 10 and a surfactant, and wherein the fluid contains less than about 15% by volume of aphrons preferably generated by the turbulence and pressure drop as the fluid exits the drill bit in the vicinity of the formation. The method of drilling a wellbore in a subterranean formation comprises continuously circulating, while drilling, such a drilling fluid.

This is a divisional of Ser. No. 09/320,375, filed on May 7, 1999 nowU.S. Pat. No. 6,390,208, which is a continuation of Ser. No.PCT/US98/02566, filed Feb. 10, 1998, which is a continuation of Ser. No.08/800,727, filed Feb. 13, 1997, now U.S. Pat. No. 5,881,826.

BACKGROUND OF THE INVENTION

Formation damage due to invasion by drilling fluids is a well-knownproblem. Many zones contain formation clays which hydrate when incontact with water such as the filtrate from drilling fluids. Thesehydrated clays tend to block the producing zones, primarily sands, sothat oil and gas cannot move to the borehole and be produced.

These zones are also damaged by solids which are carried into theopenings with the fluid. The movement of drilling fluids and filtratethrough these openings also causes the dislodging and migration ofsolids in place in the formation. These solids can lodge and blockmovement of produced hydrocarbons.

Invasion is caused by the differential pressure of the hydrostaticcolumn which is generally greater than the formation pressure,especially in low pressure or depleted zones. Invasion is also due tothe openings in the rock and the ability of fluids to move through therock, the porosity and permeability of the zone.

Because of this differential pressure, drillers have long used filtratecontrol mechanisms to control the movement of drilling fluids andfiltrate into and through the formation openings. This mechanisminvolves adding particles to the drilling fluid which are then depositedonto the borehole wall while circulating and drilling. These particlesare generally some combination of bentonite, starch, lignins, polymers,barite, and drilled solids. They are used to plug and seal the boreholedue to the particle size and shape, and some control is also due to theviscosity of the filtrate when water-soluble polymers are used. Althoughthis wallcake forms a semipermeable barrier, some filtrate moves throughand into the zone both before and after the wallcake is formed.

Wallcake control, then is not complete and some filtrate water isallowed to contact the producing zone. Another disadvantage of wallcakemud is that when filtrate moves through, the solids are screened out andleft in the cake. This causes the cake to become thicker and can lead todifferential sticking of the drill string.

More recent technology has seen the development of Low Shear RateViscosity (LSRV) fluids. LSRV is created by the addition of specializedpolymers to water or brines to form a drilling fluid. These polymershave a unique ability to create extremely high viscosity at very lowshear rates. These LSRV fluids have been widely used because of theircarrying capacity and solids suspension ability. They have been acceptedas a way to minimize cuttings bed formation in high angle and horizontalwells, and as a way to reduce barite sag in high weight muds.

Recent studies and field experience indicate that this LSRV is helpfulin controlling the invasion of drilling fluids and filtrate by creatinga high resistance to movement into the formation openings. Since thefluid moves at a very slow rate, viscosity becomes very high, and thedrilling fluid is contained within the borehole with a very slightpenetration. This has been beneficial in protecting the zones fromdamage as well as reducing differential sticking in these fluids. Seefor example the article entitled “Drill-In Fluids Improve High AngleWell Production”, Supplement to the Petroleum Engineer International,March, 1995.

Lost circulation is also a severe problem in rotary drilling. Lostcirculation occurs when the differential pressure of the hydrostaticcolumn is much greater than formation pressure. The openings in the rockare able to accept and store drilling fluid so that none is returned tosurface for recirculation. The fluid is lost downhole and can become anexpensive and dangerous problem. Lost circulation can lead to holeinstability, stuck drill pipe, and loss of well control. At the least,it halts drilling operations and requires expensive replacement volumeto be used.

In addition to the fluid volume being lost, expensive lost circulationmaterials (LCM) are required. These are usually fibrous, granular, orflake materials such as can fibers, wood fibers, cottonseed hulls, nuthulls, mica, cellophane, and many other materials. These LCM materialsare added to the fluid system so that they may be carried into the losszone and lodge to form a bridge on which other materials may begin tobuild and seal. These LCM materials themselves are damaging to thezones, and because they must be carried many times in the drilling fluidto maintain circulation, solids removal is halted and high solids mudresults.

Methods of correcting lost circulation of drilling fluids by aeratingthe drilling fluids are set forth in U.S. Pat. No. 2,818,230 (Davis) andU.S. Pat. No. 4,155,410 (Jackson).

The use of underbalanced drilling has increased as the development oflow pressure formations has acquired more importance. Horizontaldrilling, in particular, has increased the need to drill across zonesthat are not only low pressure, but highly fractured or permeable. Theexposure of numerous fractures or openings having low formationpressures has increased the problem of lost circulation and formationinvasion. The necessity of down hole tools many times preclude the useof bridging materials to stop these losses. This has led to the use ofunderbalanced drilling techniques to control the losses and invasion ofthese zones. Some of these techniques include the use of air, mist, andfoam drilling fluids. Problems with these fluids include hole cleaning,control of formation fluids, corrosion, and requirements for expensive,often hard to get equipment such as compressors and boosters. Suchfluids are not recirculateable and must be constantly generated as thedrilling proceeds.

SUMMARY OF THE INVENTION

A new fluid technique combines the use of low shear rate viscositygenerating polymers with surfactants to form colloidal gas aphrons at aconcentration less than about 15% by volume in a re-circulateable welldrilling and servicing fluid. The aphrons use encapsulated air availablein most circulating fluids. The aphrons reduce the density of the fluidand provide a means of bridging and sealing of the formations contactedby the fluid as the bubbles expand to fill the openings exposed whiledrilling. The low shear rate polymers strengthen the microbubble andalso provide a resistance to movement within the formation so thatlosses of fluid are substantially reduced as the formation is beingdrilled. In this way, lost circulation is prevented. Any fluid whichenters the formation is clean and essentially solids-free such thatdamage of the formation is significantly less than withsolids-containing fluids. Since no solids or particles are involved inthis method, solids removal equipment can be used to keep the fluid asclean as possible.

It is an object of this invention to provide recirculateable welldrilling and servicing fluids which have an enhanced low shear rateviscosity (hereinafter abbreviated to “ELSRV”) containing aphrons.

It is another object of this invention to provide a method of bridgingand sealing subterranean formations at the surface of a borehole duringwell drilling and servicing operations.

These and other objects of the invention will be obvious to one skilledin the art upon reading this specification and claims.

The process can comprise, consist essentially of, or consist of thestated steps with the stated materials. The compositions can comprise,consist essentially of, or consist of the stated materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The well drilling and servicing fluids of this invention comprise anaqueous liquid having a water-soluble polymer hydrated therein and asurfactant. The polymers useful in the ELSRV fluids of this inventionare such that the ELSRV fluids have a “thixotropic index” of at least10, wherein the thixotropic index is the ratio of the Brookfieldviscosity at 0.5 rpm to the Brookfield viscosity at 100 rpm. Thethixotropic index is indicative of the shear thinning characteristics ofthe fluid.

The base aqueous fluid in which the low shear rate modifying polymer ishydrated may be any aqueous liquid which is compatible with the polymer.Thus the base liquid may be fresh water, or a brine containing solublesalts such as sodium chloride, potassium chloride, calcium chloride,sodium bromide, potassium bromide, calcium bromide, zinc bromide, sodiumformate, potassium formate, cesium formate, and the like. The brine maycontain one or more soluble salts at any desired concentration up tosaturation.

The polymers useful in the ELSRV fluids of this invention comprise anywater-soluble polymer which increases the low shear rate viscosity ofthe fluid to produce a fluid exhibiting a high yield stress, shearthinning behavior. Particularly useful are biopolymers produced by theaction of bacteria, fungi, or other microorganisms on a suitablesubstrate. Exemplary biopolymers are the polysaccharides produced by theaction of Xanthomonas compestris bacteria which are known as xanthangums. These are available commercially from several sources including:Kelco Oil Field Group, Inc., under the trademarks “Xanvis” and “Kelzan”;Rhone-Poulenc Chimie Fine, under the trademark “Rhodopol 23-p”; PfizerInc., under the trademark “Flocon 4800C”; Shell International ChemicalCompany of London, U.K., under the trademark “Shellflo ZA”; and DrillingSpecialties Company, under the trademark “Flowzan.” See for example U.S.Pat. No. 4,299,825 and U.S. Pat. No. 4,758,356, each incorporated hereinby reference. Other biopolymers useful in the fluids of this inventionare the so-called welan gums produced by fermentation with amicroorganism of the genus Alcaligenes. See for example U.S. Pat. No.4,342,866, incorporated herein by reference. Gellan gums are disclosedin U.S. Pat. No. 4,503,084, incorporated herein by reference.Schleroglucan polysaccharides produced by fungi of the genus sclerotiumare disclosed in U.S. Pat. No. 3,301,848, incorporated herein byreference. Commercially available schleroglucan is sold under thetrademarks “Polytran” from the Pillsbury Company and “Actigum CS-11”from CECA S.A. Succinoglycan polysaccharides are produced by cultivatinga slime-forming species of Pseudomonas, Rhizobium, Alcaligenes orAgrobacterium, e.g., Pseudomonas sp. NCIB 11264, Pseudomonas sp. NCIB11592 or Agrobacterium radiobacter NCIB 11883, or mutants thereof, asdescribed in European Patent No. A40445 or A138255. Commerciallyavailable succinoglycan biopolymer is sold by Shell InternationalChemical Company of London, U.K., under the trademark “Shellflo-S”.

The minimum concentration of the polymer required to increase the lowshear rate viscosity of the fluid can be determined by routine testing.Thus the minimum concentration will be an amount sufficient to impart tothe fluid the desired low shear rate viscosity. Generally the fluidswill contain a concentration from about 0.7 kg/m³ (0.25 ppb) to about11.4 kg/m³ (4 ppb), preferably from about 1.4 kg/m³ (0.5 ppb) to about7.1 kg/m³ (2.5 ppb).

The water base borehole fluids of this invention generally may containmaterials well known in the art to provide various characteristics orproperties to the fluid. Thus the fluids may contain one or moreviscosifiers or suspending agents in addition to the polysacchariderequired, weighting agents, corrosion inhibitors, soluble salts,biocides, fungicides, seepage loss control additives, bridging agents,deflocculants, lubricity additives, shale control additives, and otheradditives as desired.

The borehole fluids may contain one or more materials which function asencapsulating or fluid loss control additives to further restrict theentry of liquid from the fluid to the contacted shale. Representativematerials known in the art include partially solubilized starch,gelatinized starch, starch derivatives, cellulose derivatives, humicacid salts (lignite salts), lignosulfonates, gums, synthetic watersoluble polymers, and mixtures thereof.

The fluids of this invention should have a pH in the range from about7.0 to about 11, preferably from 8 to about 10.5. The pH can be obtainedas is well known in the art by the addition of bases to the fluid, suchas potassium hydroxide, potassium carbonate, potassium humate, sodiumhydroxide, sodium carbonate, sodium humate, magnesium oxide, calciumhydroxide, zinc oxide, and mixtures thereof. The preferred base ismagnesium oxide.

The surfactants useful in the present invention to create the aphronsmust be compatible with the polymers present in the fluid to create thedesired low shear rate viscosity. Thus the surfactants will generally benon-ionic or anionic. A test procedure has been devised to determine ifa surfactant can be used in the present invention to generate theaphrons. The procedure is as follows:

To a low temperature, low pressure API filtration cell (API RecommendedPractice 13 B-1), the cylindrical body of which is made from Plexiglasof thickness 0.5 inch (1.3 centimeters), is added 200 grams of sandhaving a particle size in the range from 50 mesh to 70 mesh (297 μm to210 μm). This provides a sand bed depth of 2.1 centimeters. No filterpaper is used in the cell. 350 cc of the fluid to be tested is slowlyadded to the cell, the cell assembled, and 100 psi nitrogen pressureapplied. The pressure is released after the nitrogen blows through thebed for 30 seconds. Upon releasing the pressure the sand bed will expandin volume/height as the bubbles in the sand bed expand. The expansion isnot even, and an average increase in height of the bed as measured atthe cell wall and at the center of the sand bed is obtained. Surfactantswhich increase the sand bed by at least 50% are considered to bepreferred for the generation of aphrons in the present invention. TestFluid: contains 1.5 pounds per 42 gallon barrel (4.285 kg/m³) of wellhydrated xanthan gum in water and 1 pound per 42 gallon barrel (2.857kg/m³) of surfactant to be tested. The surfactant is dispersed in thexanthan gum dispersion by spatulation to prevent the generation of afoam. Solid surfactants are first dissolved in an appropriate waterdispersible or soluble solvent before adding them to the xanthan gumdispersion.

The book by Felix Sebba entitled “Foams and Biliquid Foams—Aphrons”,John Wiley & Sons, 1987, incorporated herein by reference, is anexcellent source on the preparation and properties of microbubbles.

An aphron is made up of a core which is often spherical of an internalphase, usually liquid or gas, encapsulated in a thin aqueous shell. Thisshell contains surfactant molecules so positioned that they produce aneffective barrier against coalescence with adjacent aphrons.

The aphrons when first generated contain a wide size distributionranging up to about 200 μm in diameter. At atmospheric pressure, theaphrons of very small diameter diminish very rapidly leaving aphrons inthe 25 μm to about 200 μm size range. This is due to the excess pressurewithin the aphrons which increases as the diameter of the aphronsdecreases. Thus the smaller aphrons will tend to diminish in size bytransferring their gas to the larger ones which would have a lowerexcess pressure.

In the case of the aphron-containing well drilling and servicing fluidsof the present invention, the aphrons are generated downhole as thefluid exits the drilling bit. The fluid is under considerable pressurecomposed of hydrostatic as well as pressure loss created by thecirculating system. It is believed that this fluid pressure compensatesfor the excess pressure within the aphrons such that the aphrons smallerthan about 25 μm are stabilized for a period of time until they arecirculated up the borehole. The aphrons thus are able to penetratewithin the pore spaces of the exposed formation where they can expand,because of the lower pore pressure within the formation, and seal thepore spaces from the entry of any fluid. Microfractures and the likewill be filed with aphrons which likewise expand within the formation toseal the microfractures.

Increases in vapor pressure due to pressure drops, temperatureincreases, and cavitation are common in downhole conditions. Certainsolvents which may be present in the fluid may also effect vaporpressure to provide gasses needed to form aphrons.

Aphrons large enough to be seen without magnification can be visuallyobserved in the fluid as it flows from the borehole into the surfaceholding tanks (“pits”) before being recirculated. Generally the fluidflows across a screen to remove the drill cuttings. Screens as fine as200 mesh (74 μm screen openings) can be used with the fluids of thepresent invention. Aphrons greater than the screen size will be removedfrom the fluid. If desired, the particle size of the aphrons in thefluid can be determined with various particle size analyzers which arecommercially available. See for example the following articles: (1)“Microbubbles: Generation and Interaction with Colloid Particles”, JamesB. Melville and Egon Matijevic, Chapter 14 in “Foams”, R. J. Akers,editor, Academic Press, 1976; (2) “Separation of Organic Dyes fromWastewater by Using Colloidal Gas Aphrons”, D. Roy, K. T. Valsaraj, andS. A. Kottai, Separation Science and Technology, 27(5), pp. 573-588(1992). These articles are incorporated herein by reference.

Upon being recirculated down the drill string and through the bitadditional aphrons are generated provided the concentration of thesurfactant is sufficient. It is desirable to add additional surfactantto the fluid either continuously or intermittently until the desiredquantity of aphrons is produced.

The quantity of aphrons in the fluid depends on the density required.Generally, the fluid will contain less than 15% by volume of aphrons.Thus the density of the circulating fluid can be monitored on thesurface and additional surfactant added as necessary to maintain thedesired density, if the density is too high, and weight material may beadded if the density is too low. The quantity of aphrons in the fluidcan be determined by adding a known quantity of a defoamer or otherchemical to destabilize the surfactant-containing shells surrounding theaphrons. Measurement of the change in volume of the fluid will indicatethe volume % of aphrons in the fluid.

The concentration of surfactant required in any case is less than thecritical micelle concentration (CMC) of the surfactant. Generally aconcentration of surfactant from about 0.015% by volume to about 0.15%by volume, depending on the particular surfactant present in the fluid,is required, preferably from about 0.03% to about 0.1% by volumeassuming the surfactant contains about 80% by weight solids.

If desired, the aphrons can be generated on the surface using theprocedures and equipment set forth in the following U.S. patents,incorporated herein by reference: Sebba U.S. Pat. No. 3,900,420 andMichelsen U.S. Pat. No. 5,314,644. The well drilling and servicing fluidcontaining the aphrons can then be continuously circulated in theborehole.

The so-called water-soluble polymer present in the fluid to enhance thelow shear rate viscosity of the fluid also helps to stabilize theaphrons, thus helping to prevent their coalescence.

It is preferred that the surfactant be added to the drilling and wellservicing fluid under pressure by pumping the surfactant into the fluid.

If necessary, air or other gas can be incorporated into the fluid toentrain more gas for forming the aphrons as the fluid exits the drillbit at the bottom of the borehole, provided that the fluid contains lessthan about 15% by volume of aphrons (encapsulated air or gas).

The following examples are illustrative of this invention and are not tobe regarded as limitative.

The Lost Circulation Preventative Fluid system (hereinafter sometimesreferred to as “LCPF” System) is initially prepared containing 1.5-2.0lbm/bbl (4.285-5.714 kg/m³) of xanthan gum biopolymer and 0.075% byvolume of a blend of nonionic and anionic surfactants (80%concentration, by weight, in an aqueous solution). This surfactant blendexhibited an average % increase in the height of sand bed test of 55%.The biopolymer is hydrated in the fluid, and the surfactant is injectedunder pressure into the fluid in the standpipe. The LCPF system has beenevaluated as indicated in the Examples.

The low shear rate viscosity was increased for hole cleaning and tocreate a resistance to movement into the formation, while the polymerencapsulation helped provide strength for the bubble wall surroundingthe aphrons produced downhole as the LCPF system exited the drill bit.The surfactant solution enabled the aphrons to form, reducing the fluiddensity and providing “bubble bridging” to seal off the formationsdrilled.

EXAMPLE 1

Background

A horizontal reentry well was planned in the Lodgepole formation inBillings County, North Dakota. The drilling fluid requirements wereseveral. The fluid had to have carrying capacity to carry out milledcuttings as the window was cut. It also would need lubricity andstability in carrying out the drilling operations during the build andlateral section, and the ability to provide invasion control whiledrilling the Lodgepole producing zone.

Lost circulation prevention was, of course, a necessity since the bottomhole pressure was low and the formation was fractured. Because of thedownhole tools, MWD and mud motors, no bridging materials could be usedto control losses.

Another factor was the cold weather. Freezing temperatures required somesalinity so that cut brine was used, and the resulting base fluidweighted over 9.3 ppg. The fluid then had to provide a measure of lostcirculation prevention and invasion control due to this overbalancecondition.

For these reasons, the well was planned using the LCPF system.

Application

The LCPF System was prepared and circulated in the borehole and drillingcommenced. The milling, kickoff, and build operations were done with noproblems. The zone was drilled with the LCPF system containing about 7%by volume aphrons having a density of 8.7 ppg. This low density, alongwith the invasion control properties of the system allowed the operatorto drill the zone successfully.

The lateral was drilled as planned with no losses and with excellenthole conditions.

EXAMPLE 2

Background

A well was being drilled in the Sprayberry area of West Texas. Severelost circulation was common while drilling in this area. It wasnecessary to carry 12 lbm-bbl (34.3 kg/m³) or more lost circulationmaterial and bypass the solids removal equipment. Whenever lostcirculation material content dropped, losses would recur.

Mud problems and poor hole conditions were common due to the buildup ofsolids and a decision was made to replace the existing system with theLCPF System.

Application

The LCPF System was prepared and circulated in the borehole to displacethe fluid in the hole and to create aphrons in the fluid. The aphrons,about 12% by volume, helped to reduce the density from 9.2 to 8.2 ppgand formed a “Bubble Bridge” helping stop fluid movement into the losszone. Solids removal was resumed and the well was drilled to total depthwith no further losses. A subsequent well was being drilled in the areausing the LCPF System with no losses and no mud problems.

EXAMPLE 3

Two re-entry wells were drilled in the North Texas area into the reefportion of the dolomitic zone. This formation was highly vugular withlarge, interconnected openings. Severe losses had been experienced inthis zone.

A typical procedure was to drill into the zone, and if it was present,complete returns were lost. To regain circulation meant pumping awaylarge volumes of drilling muds with high concentrations of bridgingmaterials, as high as 35 lbm/bbl (100 kg/m³).

In this area, the problem was compounded by the presence of a gas capabove the reef zone requiring 9.0 ppg fluid to prevent gas entry.

After careful evaluation of the severe problems in this area, a programwas designed to provide success in drilling and evaluating these zonesby using the LCPF System.

Application

The LCPF System was prepared and drilling commenced while surfactant wasinjected. When adequate aphrons were generated in the LCPF system, thesystem was weighted up with barite to 9.0 ppg and the zone was drilledwithout any lost circulation.

Logging and completion was easily accomplished and the wells were put onproduction with no cleanup or stimulation required.

EXAMPLE 4

A horizontal well was planned in the Sisquoc formation in Santa BarbaraCo., California. Solutions to several problems were crucial to thesuccess in drilling this well.

The Sisquoc is a multi-layered, water sensitive zone containing clays,shales, and sand. Drilling horizontally across it would requireinhibition for shale stability, prevention of cuttings bed buildup inthe lateral and build section, and the ability to maintain circulationthrough the low pressure, unconsolidated sands.

Use of conventional lost circulation material was prohibited sincelogging while drilling navigation tools would be required to accuratelydrill the zone. Invasion of the sensitive zone with solids and lostcirculation material laden fluid was also discouraged.

For these reasons, the well was planned using the LCPF System.

Application

The multiple clay, shale and sand zones were drilled with a low densityLCPF System. This low density, along with the invasion controlproperties of the system allowed the operator to drill the zonesuccessfully.

The intermediate was drilled through reactive clay beds and shales whilebuilding angle to a casing point of 92° where casing was set with noproblems. Previous wells experienced severe problems drilling andrunning casing through this interval.

The lateral borehole was drilled past 800 feet (243.8 meters) with nolosses and good hole conditions. A 6⅝″ slotted liner was run to bottomwith no difficulty.

EXAMPLE 5

Surfactants were screened for use in the present invention using thetest procedure set forth hereinbefore. The average percent increase inheight of the sand bed is as follows:

Surfactant % Increase Sodium dioctyl sulfosuccinate 118.8 Chubb NationalFoam-High Expansion 96.4 Alpha olefin sulfonate 63.7 Ethoxylated2,4,7,9-tetramethyl-5-decyn-4,-diol 56.0 Linear C₉-C₁₁ alcoholethoxylates, ave. 6 moles EO/mole 56.0 TetrasodiumN-(1,2-dicarboxyethyl)-N-octadecyl sulfosuccinate 50.6 Mixture ofdiethanolamides of fatty acids 50.0 Sodium disopropyl naphthalenesulfonate 38.1 Linear C₁₂-C₁₅ alcohol ethoxylates, ave. 7 moles EO/mole38.1 Modified alkyl ether sulfate 28.6 EthoxylatedOctadecylamine-Octadecylguanidine complex 19.0 Ethoxylated (20 moles)methyl glucoside sesquistearate 19.02,4,7,9-tetramethyl-5-decyne-4,7-diol <10 Ethoxylated (1 mole) nonylphenol <10 Sodium alkyl sulfate <10 Polyoxypropylene-polyoxyethyleneblock copolymer <10

What is claimed is:
 1. A well drilling and servicing fluid which can becontinuously circulated in a borehole comprising an aqueous liquid, apolymer which increases the low shear rate viscosity of the fluid to theextent that the thixotropic index of the fluid is at least 10, asurfactant, and aphrons which are generated by encapsulation of gas inthe fluid by a thin aqueous surfactant-containing shell wherein thesurfactant molecules are so positioned that they produce an effectivebarrier against coalescence with adjacent aphrons, the fluid containingless than about 11% by volume of aphrons.
 2. The well drilling andservicing fluid of claim 1 wherein the polymer is a biopolymer.
 3. Thewell drilling and servicing fluid of claim 1 or 2 wherein the surfactantprovides an average percent expansion of a sand bed of the least about50% when evaluated according to the following test procedure: to a lowtemperature, low pressure API filtration cell (API Recommended Practice13 B-1), the cylindrical body of which is made from Plexiglass ofthickness 0.5 inch (1.3 centimeters) is added 200 grams of sand having aparticle size in the range from 50 mesh to 70 mesh (297 μm to 210 μm);this provides a sand bed depth of 2.1 centimeters; no filter paper isused in the cell; 350 cc of the fluid to be tested is slowly added tothe cell, the cell assembled, and 100 psi nitrogen pressure applied; thepressure is released after the nitrogen blows through the bed for 30seconds so as to form bubbles; upon releasing the pressure the sand bedwill expand in volume/height as the bubbles in the sand bed expand; theexpansion is not even, and an average increase in height of the bed asmeasured at the cell wall and at the center of the sand bed is obtained;wherein the test fluid comprises 4.285 kg/m³ of well hydrated xanthangum in water and 2.857 kg/m³ of the surfactant to be tested, wherein thesurfactant is dispersed in the xanthan gum dispersion by very low shearmixing to prevent the formation of a foam.
 4. A recirculateable drillingfluid, comprising: an aqueous liquid; a viscosifier that increases thelow shear rate viscosity of the fluid to the extent that the shearthinning index of the fluid is at least 10; a surfactant; and aphrons,wherein the aphrons comprise less than about 11% by volume of the fluid.5. A recirculateable drilling fluid according to claim 4 wherein theaphrons comprise less than about 6.5% by volume of the aphrons.
 6. Arecirculateable servicing fluid, comprising: an aqueous liquid; aviscosifier that increases the low shear rate viscosity of the fluid tothe extent that the shear thinning index of the fluid is at least 10; asurfactant; and aphrons, wherein the aphrons comprise less than about11% by volume of the fluid.
 7. A recirculateable servicing fluidaccording to claim 6 wherein the aphrons comprise less than about 6.5%by volume of the aphrons.
 8. The drilling or servicing fluid accordingto claims 4 or 6 wherein the aphrons prevent loss of excess fluid in aformation.
 9. The drilling or servicing fluid according to claims 4 or 6wherein the aphrons are generated by encapsulation of gas in the fluidby a thin aqueous surfactant-containing shell wherein the surfactantmolecules are so positioned that they produce an effective barrieragainst coalescence with adjacent aphrons.
 10. The drilling or servicingfluid according to claims 4 or 6 wherein the aphrons effectively seal aformation.
 11. The drilling or servicing fluid of claims 4 or 6 whereinthe viscosifier is a polymer.
 12. The drilling or servicing fluid ofclaim 11 wherein the polymer is a polysaccharide.
 13. The drilling orservicing fluid of claim 11 wherein the polymer is a biopolymer.
 14. Adrilling fluid, comprising: an aqueous liquid; a viscosifier thatincreases the low shear rate viscosity of the fluid to the extent thatthe thixotropic index of the fluid is at least 10; a surfactant; andaphrons.
 15. A servicing fluid, comprising: an aqueous liquid; aviscosifier that increases the low shear rate viscosity of the fluid tothe extent that the thixotropic index of the fluid is at least 10; asurfactant; and aphrons.
 16. The drilling or servicing fluid accordingto claims 14 or 15 wherein the fluid is recirculateable.
 17. Thedrilling or servicing fluid according to claims 14 or 15 wherein theaphrons comprise less than about 11% by volume of the fluid.
 18. Thedrilling or servicing fluid according to claims 14 or 15 wherein theaphrons comprise less than about 6.5% by volume of the fluid.
 19. Thedrilling or servicing fluid of claims 14 or 15 wherein the viscosifieris a polymer.
 20. The drilling fluid of claim 19 wherein the polymer isa biopolymer.
 21. The drilling fluid of claim 14 wherein the surfactantmolecules are so positioned that they produce an effective barrieragainst coalescence with adjacent aphrons.
 22. The drilling fluid ofclaim 19 wherein the aphrons comprise less than about 11% by volume ofthe fluid.
 23. The drilling fluid of claim 19 wherein the aphronscomprise less than about 6.5% by volume of the fluid.
 24. The drillingfluid of claim 19 wherein the fluid is recirculateable.
 25. The drillingfluid of claim 24 wherein the polymer increases the low shear rateviscosity of the fluid to the extent that the thixotropic index of thefluid is at least
 10. 26. The drilling fluid of claim 25 wherein theaphrons comprise less than about 11% by volume of the fluid.
 27. Thedrilling fluid of claim 25 wherein the aphrons comprise less than about6.5% by volume of the fluid.
 28. The drilling fluid of claim 14 whereinthe aphrons prevent loss of excess drilling fluid in a formation. 29.The drilling fluid of claim 14 wherein the aphrons effectively seal aformation.
 30. The servicing fluid of claim 19 wherein the polymer is abiopolymer.
 31. The servicing fluid of claim 15 wherein the surfactantmolecules are so positioned that they produce an effective barrieragainst coalescence with adjacent aphrons.
 32. The servicing fluid ofclaim 19 wherein the aphrons comprise less than about 11% by volume ofthe fluid.
 33. The servicing fluid of claim 19 wherein the aphronscomprise less than about 6.5% by volume of the fluid.
 34. The servicingfluid of claim 19 wherein the fluid is recirculateable.
 35. Theservicing fluid of claim 34 wherein the polymer increases the low shearrate viscosity of the fluid to the extent that the thixotropic index ofthe fluid is at least
 10. 36. The servicing fluid of claim 35 whereinthe aphrons comprise less than about 11% by volume of the fluid.
 37. Theservicing fluid of claim 35 wherein the aphrons comprise less than about6.5% by volume of the fluid.
 38. The servicing fluid of claim 15 whereinthe aphrons prevent loss of excess servicing fluid in a formation. 39.The servicing fluid of claim 15 wherein the aphrons effectively seal aformation.